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O. De Marco (American Museum of Natural History), M. Cohen (Berkeley), M. J. Barlow (University College London)
Hydrogen-deficient central stars of planetary nebula (PN) present a mystery and a challenge in the evolution of low mass stars after they ascend the Asymptotic Giant Branch (AGB). This class represents about 20% of all known central stars and seems to evolve as a result of an outburst event early in the life of the post-AGB star. During this event, which is thought to be similar to a helium shell outburst, ALL the hydrogen is burned or ejected, leaving a central star made of helium and carbon, which can later develop strong mass-loss.
SwSt1 is one such star. It has a very young PN, with high density and a very small apparent radius. Its star is hot (35000K) and has a substantial mass-loss. Because of its youth, this object, together with a handful of central stars, can be used to trace the evolution of this class back to their AGB ancestry and to the event that made them different from H-rich central stars. Recently-acquired spacially-resolved STIS observations, show density and abundance layering in the PN gas, which can be used to trace the object's history.
In this paper, the HST observations are analyzed with stellar non-LTE codes, to determine the stellar parameters and hence the stellar evolutionary status (effective temperature and radius). Nebular photo-ionization codes, which use the synthetic stellar atmosphere as input, are then used to model the nebular emission as a function of radius. Probing into as little as the last 500 years in the life of this class of objects using their young PN, can characterize the event that is responsible for their hydrogen-deficient nature. Mass-loss on and after the AGB is still one of the least understood events in stellar astrophysics. Progress in understanding the super-efficient mass-loss that leads to central stars stripped of hydrogen would constitute a significant advance in characterizing AGB mass-loss in general.